This invention relates generally to x-ray tubes and more particularly to x-ray tubes having a rotating x-ray target.
As is known in the art, x-ray tubes have a wide variety of applications and constructions. In one type of x-ray tube, a high-energy electron beam is emitted by a heated-filament cathode and focused onto an x-ray emissive region of an anode. The x-ray emissive region comprises a material, such as a tungsten-rhenium alloy, which emits x-rays in response to the incidence of such focused electron beam thereon. Hence, the anode is commonly referred to as the x-ray target. The incidence of the high-energy electron beam on the x-ray target generates a large amount of heat in the target which is deleterious to the target structure and hence to the operation of the x-ray tube. Hence, in conventional x-ray tubes of this type, the target anode comprises a member, such as a disk, which is mounted on a supporting shaft and rotated by a motor. The disk and supporting shaft are typically made of a high-melting-temperature metal alloy, such as titanium-zirconium-molybdenum (TZM). The electron beam is focused onto a focal track, an annular portion of a surface of the disk comprising the tungsten-rhenium x-ray emissive material. The motor rotates the target disk at high speeds, for example 10,000 RPM, thereby rotating portions of the focal track into and out of the path of the focused electron beam. The electron beam therefore is incident on only a portion of the x-ray emissive focal track at a time, allowing the remainder of the focal track to cool during the time taken to rotate such portions back into the path of the focused electron beam.
In conventional x-ray tubes, the supporting shaft comprises a flanged mounting member with a threaded stem projecting therefrom. The stem fits through an opening disposed in the center of the target disk, such disk engaging a surface of the flanged mounted member. The TZM x-ray target disk is secured to the TZM supporting shaft by clamping the disk against the flanged mounting member with a TZM nut threaded onto the stem. The target disk is then dynamically balanced on the shaft, for example by shifting weights on the underside of the disk or removing small amounts of material from the underside of the disk. While the above-described securing arrangement has performed satisfactory in some applications, in other applications the target disk has been found to slip on the supporting shaft due to the high speed rotation of the disk, thereby causing the target disk to become unbalanced on the shaft. Consequently, the unbalanced target disk vibrates during rotation, such vibration often becoming so pronounced as to destroy the x-ray tube.
In another conventional x-ray tube, the stem of the supporting shaft is unthreaded and is made from material other than TZM, such as niobium (also referred to as columbium). During manufacture, the stem and target disk are heated, thereby diffusion bonding the target to the stem and hence to the supporting shaft. While such mounting method functions satisfactorily in some applications, a stem made from material such as niobium is weaker than a stem fabricated from a refractory metal alloy, such as TZM, especially in the high temperatures commonly encountered during x-ray tube operation. Hence, at such high temperatures the diffusion bond between the stem and target disk is relatively weak, often resulting in the above-described slippage of the target on the supporting shaft, with accompanying target imbalance and vibration. Additionally, it is noted that the stress put on the diffusion bond between the stem and the target disk is tension or compression stress, which is the most destructive type of mechanical stress for a diffusion bond. Further, through extended use the size of the opening in the target disk through which the niobium stem fits expands slightly. Such expansion puts additional tension stress on the diffusion bond between the niobium stem and the target disk.